hat is, to be able to modify the composition of the
infected soil in such a way as to make it sterile as regards malaria,
without taking from it the power of furnishing products useful for the
social economy. But all the elements indispensable for obtaining such a
result fail us utterly just here. We do not yet know what ought to be, in
general terms, the composition of a soil incapable of producing malaria,
yet retaining those properties which are suitable for vegetation. When we
shall have arrived at this first stage, there will still be a long road to
travel; and the most difficult part will be to discover a practical means
of imparting this salutary composition to all the numerous varieties of
malarious soils.

Scientifically, then, in the present state of our knowledge we are unable
to affirm anything on this point. Practically, we are not much further
advanced. It is very probable that the combination of hydraulic
purification with a forced cultivation of the soil has sometimes
determined changes in its composition by which it has been rendered
sterile as regards malaria. If that has happened, it has happened by
chance, and we are unable to reproduce the result at will; for we have not
all the data which might enable us to understand how it has come about.
Most of the purifications obtained in ancient times, by means of forced
cultivation, continued during centuries, have not been definite at all,
but the production of malaria has been simply suspended. Hardly was the
regular cultivation of the fields interrupted than the production of
malaria recommenced. Among the numerous examples that I might cite in this
connection, I will limit myself to that of the Roman Campagna. This seemed
to have been made permanently healthy under the Antonii, but after the
fall of the empire it began again to produce malaria, as if the forced
cultivation through so many centuries had never been.

One might, strictly speaking, be content with such a result, and boldly
undertake forced cultivation of all malarious districts, without stopping
to ascertain whether the freedom from malaria so obtained would be
definite, or whether the production of the poison were only suspended.
Unfortunately, one is never sure of arriving at such a result, and no one
can say, _a priori_, whether the forced cultivation of a given malarious
tract will render it healthful. It must always be remembered that the
first effect of forced cultivation, which requi